On Wed, 21 Feb 2024 at 06:56, Ho, Tong <tong...@amd.com> wrote: > > On Thu, Feb 8, 2024 at 3:36 AM, Peter Maydell <peter.mayd...@linaro.org> > wrote: > > > This implementation will send the transmitted characters > > to the QEMU chardev and also loop them back into the UART > > when loopback is enabled. Similarly if we receive a character > > from the real input we will put it into the FIFO still, so > > the FIFO will get both looped-back and real input together. > > > I think we only have one other UART where loopback is implemented, > > and that is hw/char/serial.c. In that device we make loopback not > > send transmitted characters out when in loopback mode, because > > the 16550 datasheet explicitly says that's how its loopback > > mode works. The PL011 datasheet is unfortunately silent on > > this question. Do you have a real hardware PL011 that you > > can check to see whether when it is in loopback mode > > transmitted data is also sent to the output port as well > > as looped back? Similarly for input: we should check whether > > the UART continues to accept real input or if the real input > > is completely disconnected while in loopback mode. > > Hi Peter, > > Here is what I found using hardware I have access to. > > When loopback is enabled: > > 1. Receive is disconnected from the real input and > only accepts transmit from loopback. > > 2. Transmitted characters is sent to both physical > output and loopback to receive. > > #2 is also collaborated by commit message for > https://github.com/torvalds/linux/commit/734745ca > > However, the same message also suggested that > #2 may not be the case in other implementations of pl011. > > I will work on v2 to address you other comments > as well, with a property for customizing whether > transmit will send to both in loopback mode.
Thanks for checking against the hardware behaviour. I think that unless you have a need for both behaviours in loopback mode, I would be happy to just implement the same thing as the hardware you tested, and not worry about adding the property. -- PMM
